Several semiconductor devices have been proposed as a hot electron transistor (HET) using hot electrons. A first prior art is a resonant-tunneling hot electron transistor device (RHET) proposed by Yokoyama et al. (see Japanese Journal of Applied Physics Letters, Vol. 24, No. 11, p. L853, (1985))
FIG. 11 and FIGS. 12(a), 12(b), and 12(c) are views showing a device structure and a principle of operation shown in FIGS. 1 and 3 of the cited reference by Yokoyama et al. In the first prior art, after an AlGaAs layer 21 is grown on a n+-GaAs substrate 20 to be 300 nm thick, a Si doped n+-GaAs layer 22, an AlGaAs barrier layer 23, a GaAs well layer 24, an AlGaAs barrier layer 25, and a Si doped n+-GaAs layer 26 are grown to be 100 nm, 5 nm, 5.6 nm, 5 nm, and 50 nm, respectively.
The first prior art comprises a resonant-tunneling structure within an emitter region of the HET, and a device operation at 77K has been reported. The operation is as follows. When a base 12 and an emitter 14 are equipotential, electron energy of the emitter is lower than a quantum level (E1) of a quantum well 13 provided between the emitter 14 and the base 12 as shown in FIG. 12(a), so that no current flows through the emitter.
Upon a voltage being applied between the base and the emitter, the electron energy of the emitter conforms to the quantum level of the quantum well as shown in FIG. 12(b), thereby generating a resonant-tunneling effect.
More specifically, the electron energy of the emitter has a certain distribution, and, only electrons having energy that conforms to the quantum level are emitted to the base by the resonant-tunneling effect. Since the emitted electrons have high energy, they pass through the base layer at a high speed substantially without scattering (ballistic conduction), go over an energy barrier (qΦc) between a base layer and a collector barrier layer, and are injected into the collector barrier layer.
The injected electrons travel through the collector barrier layer substantially without scattering, and reach a collector layer. Since the electrons are not substantially scattered throughout the above-described process, it is expected that the device operates at a speed higher than that of a general transistor device depending on scattering and diffusion.
However, the first prior art has a problem that an operating temperature is limited at a room temperature, device gain is small, and an operation speed is not so high as expected. For example, in the first prior art, operation at 77K is reported, but the operation at a room temperature and improvement of the operation speed is not described.
Accordingly, an InP based HET operating at a room temperature (second prior art) has been reported (see IEEE Electron Device Letters, Vol. 14, No. 9, pp. 441-443, September, (1993)).
However, the second prior art has a problem that the device gain is smaller than that of a general transistor, and the operation speed is not especially high.